Turbulent Velocity Profiles and Boundary Shear in Gravel Bed Rivers

Vertical profiles of turbulent streamwise velocities in gravel bed rivers are investigated. Field measurements made at high and low flows with electronic pitot tubes show logarithmic velocity profiles to extend over much of the flow depth. For the gravel bed rivers studied the velocity at 0.6 of the total depth was generally a good indicator of depth-averaged flow velocity. An unambiguous definition of flow depth is adopted to deal with situations where the bed is uneven or moving. When hydraulic roughness Z0 is defined as a fitted parameter of a logarithmic velocity profile, the river data indicate that the profile origin displacement below the tops of roughness elements scales with Z0. No direct relation between Z0 and bed material size is evident under mobile bed conditions. For these conditions a relation between hydraulic roughness and U*2 is identified (with U* also derived as a log profile parameter). A flow resistance equation using this relation is verified by comparison with mobile bed laboratory measurements in which U* is not fitted from velocity profiles.